Photoplethysmography or photoplethysmographic (PPG) systems are typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient's tissue—where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The changing light characteristics can be measured and used to determine the blood pressure of a patient and other parameters (blood oxygen saturation SpO2, respiration rate, heart rate, etc.).
Typically PPG measurement systems include an optical sensor for releasable attachment to the tip of patient's appendage (e.g., a finger, earlobe and others). The sensor directs light signals into the appendage where the sensor is attached. Some portion of light is absorbed and a remaining portion passes through patient tissue. The intensity of light passing through the tissue is monitored by a sensor. The intensity related signals produced by the sensor are used to compute blood parameters.
Blood pressure measurement techniques are generally put in two broad classes, direct and indirect. Direct techniques of blood pressure measurement, which are also known as invasive techniques, involve a catheter to be inserted in the vascular system. The indirect techniques are non-invasive, with improved patient comfort and safety, but at the expense of accuracy.
What is needed, therefore, is a blood pressure measurement technique and apparatus that is indirect and accurate.